The invention relates to communication networks and in particular, to tone detection in communication networks.
In a communication network, a switching system performs the necessary switching to connect a call between two callers. One example of a switching system is a PBX. Switching systems eliminate the requirement to wire call devices directly to a central office or other communication network. Rather, the call devices such as telephones, computers, facsimile machines, and modems have individual connections to the switching system, which in turn provides access to the central office or other network over one or more high-speed communication paths.
Tones are utilized in communication networks to indicate specific events to communication devices. The communication device responds to the transmission of the tone to perform a programmed operation. For example, in a switching system a disconnect tone is transmitted from the network or central office when a remote caller goes on-hook. The disconnect tone indicates to the switching system that the connection between the switching system and the network or central office should be cleared. A tone detector in the switching system continually monitors incoming communications for the disconnect tone. Responsive to receiving the disconnect tone, the tone detector clears the connection.
For economic reasons a two wire circuit typically connects the call devices to the switching system and a two wire trunk connects the switching system to the network or central office. The switching system comprises a four wire circuit that includes a two wire receive path and a two wire transmit path. Hybrid circuits in the switching system interface the four wire circuit with the two wire trunk and the two wire circuit respectively. The hybrid circuits ideally should provide a perfect impedance match between the four wire circuit, the two wire trunk, and the two wire circuit. The hybrid circuits balance the impedance so that the incoming signal from the trunk is passed directly to the receive path in the four wire circuit and the outgoing signal from the call device is passed directly to the transmit path of the four wire circuit and onto the trunk with no portion of the outgoing signal appearing in the receive path. However, variations in the characteristics of hybrid circuits result in an imbalance causing portions of the outgoing signal in transmit path of the four wire circuit to appear in the receive path of the four wire circuit and be returned to the local caller as echo return.
Unfortunately, tone detectors cannot effectively detect tone transmissions when echo return is present. One example of this problem occurs during conference calls between a remote caller connected to the network or central office and two local callers connected to the switching system. When the remote caller goes on-hook, a disconnect tone transmission is transmitted to the switching system from the network or central office. The imperfect impedance at the hybrid often results in a mixed tone transmission and echo return being received in the tone detector. The combination of tone transmission and echo return confuses the tone detector causing a detection failure. The detection failure results in an active call connection even where no caller is present. Over time, the number of active call connections without callers increases until significant network resources are depleted. Network administrates must then determine which active connections actually connect to a caller and manually clear the active connections without callers.
The present invention advances the art by providing a communication circuitry configured to provide effective tone detection in communication networks. The communication circuitry could be a stand alone device or incorporated into other devices requiring reliable tone detection. The tones could be any tones transmitted over a communication network. Advantageously, the present communication circuitry improves network efficiency and increases network resource availability.
The communication circuitry comprises filtration circuitry, summation circuitry, detection circuitry, and interface circuitry. The filtration circuitry is configured to receive a communication, process the communication to generate an estimate of an echo return present in the communication, and provide the estimate of the echo return to the summation circuitry. Prior to operation, the filtration circuitry is trained to estimate the echo return from a specific device such as the hybrid. Some examples of the communication include a voice communication, data communication, or video communication. The summation circuitry is configured to receive the estimate of the echo return from the filtration circuitry, receive a tone transmission, subtract the estimate of the echo return from the tone transmission to generate a substantially pure tone, and provide the substantially pure tone to the detection circuitry. The detection circuitry is configured to receive the substantially pure tone and process the substantially pure tone to perform a programmed operation. The programmed operation could be any operation the detection circuitry is configured to perform in response to receiving a specific tone. The interface circuitry is configured to receive the communication, receive the tone transmission, provide the communication to the filtration circuitry, and provide the tone transmission to the summation circuitry.